An electro-optic material has at least two “display states,” the states differing in at least one optical property. An electro-optic material may be changed from one state to another by applying an electric field across the material. The optical property may or may not be perceptible to the human eye, and may include optical transmission, reflectance, or luminescence. For example, the optical property may be a perceptible color or shade of gray.
Electro-optic displays include the rotating bichromal member, electrochromic medium, electro-wetting, and particle-based electrophoretic types. Electrophoretic display (“EPD”) devices, sometimes referred to as “electronic paper” devices, may employ one of several different types of electro-optic technologies. Particle-based electrophoretic media include a fluid, which may be either a liquid, or a gaseous fluid. Various types of particle-based EPD devices include those using encapsulated electrophoretic, polymer-dispersed electrophoretic, and microcellular media. Another electro-optic display type similar to EPDs is the dielectrophoretic display.
Generally, an image is formed on an electro-optic display device by individually controlling the display states of a large number of small individual picture elements or display pixels. A data pixel having one or more bits defines a particular display state of a display pixel. A frame of data pixels defines an image. Commonly, the display pixels are arranged in rows and columns forming a display matrix. An exemplary electro-optic display pixel includes a layer of electro-optic material situated between a common electrode and a pixel electrode. One of the electrodes, typically the common electrode, may be transparent. The common and pixel electrodes together form a parallel plate capacitor at each display pixel, and when a potential difference exists between the electrodes, the electro-optic material situated in between the electrodes experiences the resulting electric field.
An active-matrix display includes at least one non-linear circuit element, such as a transistor, for each display pixel. An exemplary active-matrix display pixel includes a thin-film transistor having its drain terminal coupled with the pixel electrode. The gate and source terminals of the transistor are respectively coupled with a row select line and a column data line. To change the display state of the display pixel, the common electrode is placed at ground or some other suitable voltage and a row driver circuit turns on the transistor by driving a suitable voltage on the row select line. An optical-property-dependent voltage corresponding with a display state transition may then be driven on the column data line by a column driver circuit.
An electro-optic display device may have display pixels that have multiple stable display states. Display devices in this category are capable of displaying (a) multiple display states, and (b) the display states are considered stable. With respect to (a), display devices having multiple stable display states include electro-optic displays that may be referred to in the art as “bistable.” The display pixels of a bistable display have first and second stable display states. The first and second display states differ in at least one optical property, such as a perceptible color or shade of gray. For example, in the first display state, the display pixel may appear black and in the second display state, the display pixel may appear white. In addition, display devices having multiple stable display states include devices having display pixels that have three or more stable display states. Each of the multiple display states differ in at least one optical property, e.g., light, medium, and dark shades of a particular color. As another example, a display device having multiple stable states may have display pixels having display states corresponding with 4, 8, 16, 32, or 64 different shades of gray.
With respect to (b), the multiple display states of a display device may be considered to be stable, according to one definition, if the persistence of the display state with respect to display pixel drive time is sufficiently large. The display state of a display pixel may be changed by driving a drive pulse (typically a voltage pulse) on the column data line of the display pixel until the desired appearance is obtained. Alternatively, the display state of a display pixel may be changed by driving the column data line over time with a series of drive pulses regularly spaced in time. In either case, the display pixel exhibits a new display state at the conclusion of the drive time. If the new display state persists for at least several times the minimum duration of the drive time, the new display state may be considered stable. Generally, in the art, the display states of display pixels of LCDs and CRTs are not considered to be stable, whereas the display states of EPD display pixels are considered stable.
An advantage of electro-optic displays, in general, and EPD devices, in particular, is that once a display pixel has been placed in a particular display state, the pixel will maintain that display state for a long period of time—at a minimum one or more minutes and up to hours, days, months, or longer—without drawing power. EPD devices need only be refreshed when a change in the appearance of the rendered image is desired or after the brightness of the rendered image diminishes below a desired level. In contrast, other types of display technologies maintain their display state for much shorter time periods. For example, the display pixels of a liquid crystal display (“LCD”) maintain their optical appearance for less than a second. However, in comparison with other display technologies, such as LCDs, EPD devices require relatively long drive times to cause a display pixel to assume a new display state. Thus, changing an image rendered on an EPD device may take longer than desired.
EPD devices may be used in electronic readers. The electronic reader may be used to read books, newspapers, magazines, and other documents. Oftentimes, the reader of a paper document will mark the document with a pen or pencil, such as to circle something of interest or to write notes in the margin. Some readers of a paper document, such as students, highlight passages of text with a light-colored marker. In addition, many paper documents are designed with places for the reader to write information. Examples of documents of this type range from business forms to crossword puzzles. Providing the ability to “write” on a document displayed an electronic paper device would be an advantage.
LCDs and CRTs may be provided with a sensor capable of detecting pen input. A variety of technologies have been developed in which display devices are capable of receiving input from a pen or other pointing device. For example, resistive, capacitive, acoustic, and light pen touch screens are known.
In a display device with a sensor capable of detecting pen input there is a certain amount of processing time associated with displaying a pen stroke captured by the sensor. This pen stroke capture latency would add to the time required to update an image rendered on a display device. Accordingly, there is a need for methods and apparatus for minimizing pen stroke capture latency.